1. Field of the Invention
This invention relates to computer peripheral data storage devices, and in particular to a format for embedded servo track identification fields in rotating disk data storage units such as magnetic disk drives.
2. Description of Related Art
Rotating disk data storage units ("disk drives") are commonly used to store data in computer systems. Conventionally, each disk drive comprises one or more rotating disks coated with or made from magnetic or optical recording material. Generally, each disk surface is formatted into a plurality of tracks, each track comprising a plurality of physical sectors. Movable read-write transducer heads positioned proximate to each disk surface are used to record the track and sector formats, and to read data from and write data to each physical sector.
Extensive research efforts in the field of magnetic hard disk drives for many years have been directed to developing practical techniques for increasing areal recording density and improving the reliability of such drives. Improved techniques for increasing areal recording density have been an important enabling factor in the trend in this field toward smaller, yet higher capacity, disk drives.
Areal recording density is generally expressed in terms of bits per square inch (or other unit area). Analytically, areal density is a product of the track density (i.e., the number of concentric tracks per inch, or "TPI") on the surface of a disk, and the bit density (i.e., the number of bits per inch, or "BPI") that can be recorded along a particular track.
The heads of modern disk drives are generally positioned by means of a closed-loop servo system. With a small number of stacked media disks, a common design is to write servo information in "servo wedges" embedded around the surface of each disk. Such embedded servo designs may have, for example, 10 to 100 servo wedges per disk surface, dividing the tracks of each surface into a number of sectors of a circle which may be referred to as "data wedges". FIG. 1 is a top view of a prior art embedded servo disk surface showing eight servo wedges 2, eight data wedges 4, an inner track 6, and an outer track 8. However, in modern embedded servo disk drives, the number of data wedges on a track normally would be significantly higher.
In embedded servo drives, it is necessary to read track identification information while a transducer head is moving radially across the media, as well as when the transducer head is centered over a particular track. Accordingly, each servo wedge 2 contains a track identification (ID) field, TKID, to provide such track identification information. The TKID value remains constant for each servo wedge 2 of a particular track.
One requirement for encoding identification information is that the code must be readable while the transducer head is moving radially across the disk media, and possibly spanning two tracks, as shown in FIG. 2. In FIG. 2, a transducer head 10 is traversing two adjacent tracks 11a, 11b. Since the disk surface is rotating, the path of the transducer head 10 will diagonally cut across fields on adjacent tracks 11a, 11b. Hence, it is possible that the transducer head 10 may read a first part of the TKID field 12a of one track 11 a and then a second part of the TKID field 12b of the second track 11b. Accordingly, the TKID fields 12a, 12b must be unambiguously encoded in order to positively determine the radial location of the transducer head 10.
The prior art has attempted to meet these criteria by providing a binary track identification number mapped to a Gray code and encoded using di-bits (i.e., two code bits per logical bit) written in the TKID field of each servo wedge 2. As is known, a Gray code is a modified binary code in which sequential numbers are represented by expressions that differ only in one logical bit, to minimize errors. For example, the binary sequence for decimal numbers 0 through 3 and one Gray code equivalent mapping is set forth below:
______________________________________ Decimal Binary Gray Code ______________________________________ 0 00 00 1 01 01 2 10 11 3 11 10 ______________________________________
Note that only one bit position changes in the Gray code for pairs of adjacent numbers in the sequence.
In order to permit reliable reading of a TKID field across track boundaries, the encoded data must also maintain phase coherency across track boundaries. Phase coherency means that adjacent code bits on adjacent tracks must start with the same polarity of flux reversal.
Di-bits have been chosen in the past because they meet the above criteria, and have been deemed to provide a compact encoding of the necessary track ID information. The di-bit values used in the past for encoding track ID information have been as shown in FIG. 3, in which a logical "1" is encoded as two code symbols or bits comprising a negative pulse followed by a positive pulse (or vice versa), whereas a logical "0" has been encoded by two code symbols or bits comprising the lack of pulses (i.e., only a time duration defines a "0").
A significant problem in using di-bits to encode track ID information is that long runs of logical "0"s cannot be decoded reliably. A long run of logical "0"s would be encoded as no signal, which causes a problem in decoding exactly how many logical "0" bits have been encoded. Indeed, no di-bit encoded Gray code having more than two sequential logical "0"s has been used because of decoding problems. Therefore, such Gray code values have not been used for mapping track identification numbers. Unfortunately, skipping Gray code mappings of a binary sequence requires a complex mapping circuit to decode the usable Gray codes back into a simple binary code. In practice, a relatively slow lookup table ROM circuit has been used to perform such a mapping.
Moreover, the conventional prior art di-bit encoded Gray code can be characterized as a 0,4,1,2 code. Such a code allows a minimum space of zero code bits without a flux reversal (pulse), a maximum space of four code bits without a pulse, and uses two code bits to encode one logical bit. This means that such a di-bit encoded Gray code has a minimum frequency of 1/5 of the maximum frequency. This large ratio causes known problems in signal processing and data synchronization in the read channel of a disk drive.
Conventional teaching is that di-bits are a space efficient code. However, due to the above-mentioned read channel signal processing and data synchronization problems, the linear bit density of di-bits is relatively low.
Accordingly, there is a need to provide a means of encoding track ID information in an embedded servo disk drive system that permits encoding relatively long run lengths of logical "0"s, and yet meets all of the other limitations imposed upon such codes. That is, such an encoding means must be very reliable, must be phase coherent, should consume little area on the media, and should be relatively inexpensive to detect and decode.
The present invention provides such a means.